1. Field of the Invention
The present invention relates to an X-ray analyzer provided with a radiation detector including a superconducting transition edge sensor.
2. Description of the Related Art
As an X-ray analyzer capable of discriminating X-ray energy, there are energy dispersive spectroscopy (hereinafter, referred to as EDS) and wavelength dispersive spectroscopy (hereinafter, referred to as WDS).
The EDS is an X-ray detector for converting X-ray energy input to the detector into an electrical signal and calculating energy according to the level of the electrical signal. The WDS is an X-ray detector for making an X-ray monochromatic (energy discrimination) using X-ray spectroscopy and detecting the monochromatic X-ray using a proportional counter tube or the like.
As the EDS, a semiconductor detector such as an SiLi (silicon lithium) type detector, a silicon drift type detector or a germanium detector is known. For example, a silicon lithium type or silicon drift type detector is widely used for an element analyzer of an electron microscope to detect energy in a wide range of about 0.2 keV to 20 keV. However, since silicon is used in the detector, in principle, the property of the detector depends on the band gap (about 1.1 eV) of silicon, it is difficult to improve energy resolution to about 130 eV or more, and energy resolution is lower than that of the WDS by 10 times or more.
Energy resolution, which is an index indicating performance of the X-ray detector, of, for example, 130 eV means that, when X-ray is irradiated to the X-ray detector, energy may be detected with uncertainty of about 130 eV. Accordingly, as uncertainty decreases, energy resolution increases. That is, if a characteristic X-ray composed of two adjacent spectrums is detected, uncertainty decreases as energy resolution increases. When a difference in energy between two adjacent peaks is about 20 eV, in principle, the two peaks may be divided with energy resolution of about 20 eV to 30 eV.
Recently, an energy dispersive superconducting X-ray detector having the same energy resolution as the WDS has been attracting attention. Among superconducting X-ray detectors, a detector having a superconducting transition edge sensor (hereinafter, referred to as a TES) is a high-sensitivity calorimeter using rapid resistance change (e.g., temperature change is several mK and resistance change is 0.1 Ω) upon superconduction-normal conduction transition of a metal thin film. In addition, this TES is referred to as a micro calorimeter.
A sensitive area in which a TES may detect an X-ray is in an intermediate area of normal conduction and superconduction and this point is referred to as an operation point. In order to maintain the TES at this operation point, heat balance of Joule heat generated within the TES and heat escaping from the TES to a heat tank via a heat link is formed. This heat balance is expressed as shown in Equation (1) by current I flowing in the TES, operation resistance R of the TES, heat conductivity G of the heat link, the temperature T of the TES, the temperature Tb of the heat tank and the heat Pex intruded from outside. The heat Pex intruded from outside is ideally zero.IR2+Pex=G(T−Tb)  (1)
This TES analyzes a sample by detecting temperature change in the TES occurring when a fluorescent X-ray or characteristic X-ray generated from the sample by irradiation of radiation such as a primary X-ray or a primary electron beam is made incident. The TES has energy resolution higher than the other detectors and may obtain energy resolution of 10 eV or less in the characteristic X-ray of 5.9 keV, for example.
When the TES is attached to a scanning electron microscope or a transmission electron microscope, by obtaining the characteristic X-ray generated from the sample, to which an electron beam is irradiated, is obtained by the TES, the peaks of the energy spectrums of the characteristic X-ray (for example, Si-Kα, W-Mα, W-Mβ, or the like), which cannot be divided by the semiconductor type X-ray detector, can be easily divided.
The TES is a high-sensitivity calorimeter and thus requires a plurality of heat shields, for stable operation. However, since the X-ray generated from the sample need to be introduced to the TES, an X-ray window is mounted in the heat shield (see Related-Art Document 1 listed below). In the configuration disclosed in the Related-Art Document 1, X-ray windows are mounted in heat shields respectively cooled to 4K and 80K. The X-ray windows pass the X-ray to be analyzed but block visible light or infrared light which causes noise.
In addition to the heat shields, in order to form the TES as one vacuum chamber, the X-ray window having vacuum resistance is formed to shield the outer atmosphere of a room temperature. In general, as the X-ray window having vacuum resistance, an X-ray window using an organic film is used (see Related-Art Document 2 listed below). When three X-ray windows are mounted, transmittance of X-ray is significantly reduced to 60% (1 keV) to 1% (0.2 keV or less).
Related-Art Document 1: “Transition Edge Sensor-Energy Dispersive Spectrometer (TES-EDS) and Its Applications” Keiichi TANAKA, et al., IEICE TRANSACTIONS on Electronics, vol. E92-C No.3, 2009, p.334-340
Related-Art Document 2: AP X-ray Windows. [online] . MOXTEK Incorporated, 2010. [retrieved on 2015-01-19]. Retrieved from the Internet URL: http://moxtek.com/xray-product/ap-windows